PNP

1. A type of bipolar transistor consisting of
a layer of N-doped semiconductor (the "base") between two
P-doped layers (the "collector" and "emitter"). PNP
transistors are commonly operated with the emitter at ground
and the collector at a negative voltage.

In the 1960s, the germanium PNP transistor was the cheapest
and best for use at ordinary temperatures. The leakage
current from collector to base in this type of device is
larger than for the silicon transistor, and also varies more
with temperature. The effect of these deficiencies can be
lessened by proper biasing and feedback, which can make the
circuit both serviceable and reliable. Neither germanium nor
PNP transistors are as common today.

The voltages used on a PNP transistor are inverted when
compared with vacuum tubes. Further, the behaviour of
vacuum tubes is usually described in terms of voltages
whereas transistors are better described in terms of
current.